1. Field of the Invention
This invention relates to current ballasts for fluorescent lamps and more particularly relates to the application of saturable reactor type ballasts as universal ballasts for powering a wide range of lamp wattage loads.
2. State of the Prior Art
Fluorescent lamps are low pressure mercury arc discharge devices. A filament at each end of a sealed lamp tube is heated by a filament current, and a glow discharge is sustained by a sufficiently high lamp voltage applied across the lamp tube. Such a fluorescent lamp behaves as a negative impedance in that, as current increases through the lamp, its impedance decreases. Consequently, as the arc current is initially established between the filaments, the current must be externally limited to a safe operating level to avoid destruction of the lamp. A ballast is designed to supply the necessary lamp and filament voltages while limiting the arc current across the lamp to a level which provides optimum light output without damaging the lamp tube.
Many ballasts have been devised, which generally may be classified as magnetic ballasts and solid state ballasts. Magnetic ballast tend to be simple and rely on inductance effects for limiting the arc current through a fluorescent lamp. In its simplest form, a magnetic ballast is a single coil inductor wound on a magnetic core and connected in series with the lamp tube. Solid state ballasts have evolved into highly sophisticated electronic power circuits which rely on active semiconductor devices to provide very close regulation of both current and voltage through the lamp tube.
Simple magnetic ballasts tend to exhibit limited load current regulation. This means that the electrical characteristics of the ballast must be tailored to the specific lamp load wattage to be powered, e.g. 15 Watt, 30 Watt, 40 Watt, etc. The simplest ballasts such as the single series inductor type ballasts are typically designed for a specific lamp tube wattage and different ballasts are available for each commercially available lamp tube size. The more complex ballasts, on the other hand, have wider load current regulation capabilities, and a single ballast may be interchangeably connected for powering lamp loads which may vary by a factor of 3 or even 4. For example, a high performance electronic ballast may regulate lamp current to within 1 or 2 percent over a lamp load range of 4 to 80 Watt. The more elaborate ballasts typically feature a dimming control for adjusting the light output between a bright level and a subdued light output.
One particularly demanding ballast application is in fluorescent lighting of aircraft interiors, particularly the passenger cabin of large jet liners. In such applications, it is specially desirable that the ballast be of small size and light weight in order to improve the aircraft's passenger or cargo carrying capacity. Additionally, aircraft lighting ballasts must meet particularly stringent limits on spuriously generated electromagnetic interference (EMI) which can interfere with the aircraft's sensitive radio communication and navigation equipment. Low EMI operation is more readily achieved with the simpler inductive type ballasts which do not incorporate non-linear or switching devices, either of which is conducive to the generation of higher order harmonics of the aircraft power line frequency, and high frequency noise in general. Simple ballasts are, of course, desirable from an economic viewpoint, in that their cost is generally lower than that of the more sophisticated, solid state ballasts. On the other hand, aircraft lighting installations typically include lamp fixtures of rather widely different lamp wattage. The long, tubular interior of the passenger cabin is normally lit by fluorescent strips along the ceiling center and on each of the side walls of the cabin. The cabin interior is divided into segments of unequal length by transverse bulkheads and partitions necessitated by lavatories, galleys and other features and installations of the aircraft. As a result, the lighting strips are similarly divided into segments of uneven lengths which include lamp tubes of different lengths as needed to make up the required segments. Because of this and other considerations a typical large airliner requires ballasts capable of powering fluorescent lamp loads ranging from e.g. 14 Watt to as much as 80 Watt. Since a substantial number of these ballasts are required in a large airliner, it is desirable to use a single ballast type for all of the fluorescent lamp fixtures in order to standardize the ballast throughout the aircraft cabin with a view to simplifying the aircraft's design, construction and subsequent maintenance. Aircraft lighting also requires that the fluorescent lamps be dimmable from a bright light output level to a subdued level, so that cabin illumination can be adjusted by the crew to suit various stages of a flight. Bright illumination is needed prior to and during take-off and landing, for example, while dim illumination is desirable during sleep periods or while screening in-flight movies.
What is needed therefore is a ballast of simple, low cost configuration which features good load current regulation over a relatively wide range of lamp load wattage, and which is dimmable for adjusting the light output of the fluorescent lamps.
One type of simple, dimmable magnetic ballast is the saturable reactor ballast. A saturable reactor ballast consists of a ballasting inductor wound on the outer legs of an E-type magnetic core and connected in series with the fluorescent lamp load, and a control winding on the center leg of the same magnetic core. A control current through the control winding shifts the degree of magnetization of the core and consequently changes the effective inductance of the ballasting inductor. In practice, the ballasting inductor windings and the magnetic core are selected so that in the absence of a control current the ballasting inductance limits the arc current through the lamp load to a dim light output level. For full lamp brightness a D.C. current is applied through the control winding, the core is biased along the hysteresis curve towards magnetic saturation to maintain a degree of magnetization of the core which is then driven into magnetic saturation through some portion of every cycle of the A.C. lamp voltage through the ballasting inductor, in effect reducing the inductance of the ballasting inductor. As a result, a greater arc current is delivered to the lamp load, producing a brighter light output. As the current through the control winding is varied continuously, so the light output of the lamp load can be continuously adjusted from a dim level to full brightness.
Although saturable reactor ballasts have been known for many years, consistent industry practice has been to design and manufacture these ballasts for use with specific lamp load wattages. For example, Bruce Industries, of Dayton, Nev., owner of this application, has sold saturable reactor ballasts for over ten years as their 03980-** series in wattage ratings of 14/15/20, 30, 40, 2.times.30 and 2.times.40 Watt. This series of ballasts has always been installed in commercial passenger aircraft in accordance with their specified load rating.